Implantable cardiac stimulator with safe noise mode

ABSTRACT

An implantable medical device for electrically stimulating the heart to beat generally includes a processor, a plurality of electrodes, a sense amplifier, a pulse generator, and a heart status monitor. The processor can determine when the patient has entered an environment of high electromagnetic interference. When this occurs, the processor forces the implantable device into a safe noise mode. While in the same noise mode (which preferably continues while the patient is experiencing the electromagnetic interference), the implantable device paces the heart on demand and inhibits pacing during the vulnerable period. The processor determines when the vulnerable period is occurring and when the heart needs to be paced by monitoring a status signal from the heart status monitor. The status signal generated by the heart status monitor preferably is not sensitive to the electromagnetic interference, and thus the processor can determine the bio-mechanical state of the heart during a cardiac cycle even in the face of high electromagnetic interference. The heart status monitor preferably includes an impedance measurement circuit, but may include any type of cardiac sensor that can generate a status signal from which the processor can determine the beginning and ending of the vulnerable period. Accordingly, even during a period of high electromagnetic interference, the implantable device can provide on demand pacing support to the patient.

This application is a Division of U.S. application Ser. No. 09/012,854filed Jan. 23, 1998, now issued as U.S. Pat. No. 5,978,710, which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to cardiac stimulating devices.More particularly, the present invention relates to an implantablecardiac pacemaker or cardioverter/defibrillator with a safe mode ofoperation during the occurrence of externally generated noise orinterference.

2. Description of the Related Art

In the normal human heart, illustrated in FIG. 1, the sinus (orsinoatrial (SA)) node generally located near the junction of thesuperior vena cava and the right atrium constitutes the primary naturalpacemaker by which rhythmic electrical excitation is developed. Thecardiac impulse arising from the sinus node is transmitted to the twoatrial chambers (or atria) at the right and left sides of the heart. Inresponse to excitation from the SA node, the atria contract, pumpingblood from those chambers into the respective ventricular chambers (orventricles). The impulse is transmitted to the ventricles through theatrioventricular (AV) node, and via a conduction system comprising thebundle of His, or common bundle, the right and left bundle branches, andthe Purkinje fibers. The transmitted impulse causes the ventricles tocontract, the right ventricle pumping unoxygenated blood through thepulmonary artery to the lungs, and the left ventricle pumping oxygenated(arterial) blood through the aorta and the lesser arteries to the body.The right atrium receives the unoxygenated (venous) blood. The bloodoxygenated by the lungs is carried via the pulmonary veins to the leftatrium.

This action is repeated in a rhythmic cardiac cycle in which the atrialand ventricular chambers alternately contract and pump, then relax andfill. Four one-way valves, between the atrial and ventricular chambersin the right and left sides of the heart (the tricuspid valve and themitral valve, respectively), and at the exits of the right and leftventricles (the pulmonic and aortic valves, respectively, not shown)prevent backflow of the blood as it moves through the heart and thecirculatory system.

The sinus node is spontaneously rhythmic, and the cardiac rhythm itgenerates is termed normal sinus rhythm (“NSR”) or simply sinus rhythm.This capacity to produce spontaneous cardiac impulses is calledrhythmicity, or automaticity. Certain other cardiac tissues possessrhythmicity and hence constitute secondary natural pacemakers, but thesinus node is the primary natural pacemaker because it spontaneouslygenerates electrical pulses at a faster rate. The secondary pacemakerstend to be inhibited by the more rapid rate at which impulses aregenerated by the sinus node.

If the body's natural pacemaker performs correctly, blood is oxygenatedin the lungs and efficiently pumped by the heart to the body'soxygen-demanding tissues. However, when the body's natural pacemakermalfunctions, an implantable pacemaker often is required to properlystimulate the heart. Disruption of the natural pacemaking andpropagation system as a result of aging or disease is commonly treatedby artificial cardiac pacing, by which rhythmic electrical dischargesare applied to the heart at a desired rate from an artificial pacemaker.An artificial pacemaker (or “pacer” as it is commonly described) is amedical device which includes an electronics assembly and one or moreleads connecting the electronics assembly to the heart. Electrodes onthe distal end of the leads include an exposed conducting surfaceadjacent to or in contact with the heart tissue. The pacemaker deliverselectrical pulses via the electrodes to the patient's heart in order tostimulate the heart to contract and beat at a desired rate.

Pacemakers originally were designed to operate asynchronously. Thatmeant the pacemaker emitted an electrical pulse that was delivered tothe heart through the pacemaker's electrodes at a constant rate.Asynchronous pacemakers paced the heart at a constant, preselected rategenerally thought to be sufficient for the particular patient (e.g., 70pulses per minute). This type of pacing protocol, however, unnecessarilyexpended the energy of the pacemaker's battery (which has a limitedlife) because the hearts of many patients were capable of beating ontheir own, at least occasionally, without the need for an artificiallygenerated pacemaker pacing pulse. Thus, an asynchronous pacemaker mayexpend energy pacing the heart at a time when the heart's naturalpacemaker and conduction system are functioning properly and not in needof artificial stimuli.

Pacemakers today typically are provided with the capability to determinewhether the heart is able to beat on its own, and if so, the pacemakerwill not pace the heart. If, however, the heart cannot beat on its own,the modern pacemaker will pace the heart instead. This type of pacemakeris referred to as a “demand” pacemaker because pacing pulses aregenerated by the pacemaker only as needed by the heart (i.e., on“demand”).

Some patients have disease processes that may cause an “arrhythmia,”which is an abnormal cardiac rhythm. For some of these patients thearrythmias may be characterized by an excessively slow heart rate(termed “bradycardia”) or an excessively fast and irregular heart rate(termed “tachyarrhythmia”). Tachyarrhythmia may degenerate intofibrillation in which the affected cardiac chamber merely quivers andloses all of its blood pumping capability. If the fibrillation conditionoccurs in a ventricular chamber of the heart (a condition commonlycalled “ventricular fibrillation”)₁ the patient will normally die withinminutes. In patients undergoing pacing therapy, a tachyarrhythmiahopefully can be terminated with an antitachyarrhythmia pacing protocolwhich generally includes a fast pacing rate to interfere with the focusof the arrhythmia. If antitachyarrhythmia pacing does not stop thearrhythmia, a defibrillation pulse is necessary to terminate thearrhythmia.

Patients that are susceptible to tachyarrhythmias are candidates for animplantable cardioverter/defibrillator (“ICD”) which is a device thatsenses the onset of tachyarrhythmias and generates antitachyarrhythmiapacing pulses and, if needed, a subsequent defibrillation pulse toterminate the arrhythmia. Patients that require an ICD also generallyrequire pacing support such as that provided by pacemakers. Thus, inaddition to a defibrillation capability, an ICD typically also includesa pacing capability to pace the heart on demand.

In some patients with pacemakers/ICD's observed that the pacemaker/ICDitself has demonstrated the propensity to induce a tachyarrhythmia whichmay degenerate into a fatal fibrillation. The reason for this phenomenoncan be described with respect to FIG. 2 in which a ventricularrepolarization wave (during which the ventricle relaxes aftercontracting) is detected by the surface electrocardiogram (“ECG”) as theso-called “T-wave”. The T-wave occurs approximately 150-400 millisecondsafter the occurrence of the ventricular depolarization wave shown on thesurface ECG as the QRS complex. Around the time the T-wave is detected,various portions of the ventricles are undergoing repolarization, and assuch are not sensitive to stimulation. This period of time in whichcardiac tissue is not sensitive to electrical stimulation is called therefractory period.

If pacing occurs during the refractory period, a slowly propagatingaction potential initiated by tissue which is sensitive to stimulationmay cause the stimulation of tissue which was not viable for stimulationat the time the original stimulus was generated. This propagating wavemay later reach the tissue which was originally stimulated at a timewhen it has already repolarized, and thus cause its depolarization anew.If this sequence of events occurs, a reentrant loop is created whichcauses the ventricle to beat at a rate determined by the period of thereentrant loop. This sequence causes what is known as a “reentranttachycardia,” and may degenerate into fibrillation. The period of timeduring which a pacing pulse may cause tachycardia is referred to as the“vulnerable period,” and an ICD should avoid pacing the heart during thevulnerable period. The implanted device determines the vulnerable periodby monitoring the electrical activity of the heart.

A conventional pacemaker or ICD generally includes a sense circuit formonitoring the electrical activity of the heart. The sense circuitusually includes a highly sensitive amplifier. The electrodes and leadsof the implanted device may act as antennae and pick up electromagneticsignals that have non-cardiac sources, including even signals generatedfrom a source external to the body. Such sources of external signalsgenerally are referred to as “electromagnetic interference” (“EMI”).Sources of EMI include metal detectors such as are used in airports,welders, radio transmitters, microwave ovens, etc. The electricalsignals conducted to the implanted device from the electrodes implantedin the heart may thus include EMI superimposed on the heart's naturalcardiac signal. The EMI component of the signal represents noise andpreferably is ignored.

Although the implanted device usually has some filtering circuits forattenuating noise superimposed on a cardiac signal, in some situationsthe noise component may be such that the device's filters cannotadequately eliminate the noise. If a patient with an ICD walks through ametal detector, for example, the resulting EMI signal may overwhelm thecardiac signal picked up by the electrodes. Although the implanteddevice may be able to determine that it is receiving an excessive amountof noise, the device may be unable to extract the true cardiac signalfrom the noise. Because the true cardiac electrical signal cannot beaccurately ascertained, the implanted device can not determine when thevulnerable period of each cardiac cycle is occurring. Such devices arethus often supplied with a “noise mode” of operation in which the deviceattempts to respond to the noise in some appropriate manner.

Up to now, pacemaker and ICD designers have been faced with a dilemma.If the implantable device is in a high noise field and the devicediscontinues pacing to avoid pacing during the vulnerable period, thepatient may suffer severe harm or death if the patient indeed neededpacing support. On the other hand, if the device does provide pacingduring a high noise event, the pacing will have to be performedasynchronously (fixed rate) rather than on demand because the noisedisrupts the device's sensing ability to determine when the patientneeds a pacing pulse. However, if the pacemaker or ICD providesasynchronous pacing support because the patient may need it, a pacingpulse may occur during the vulnerable period and the patients heart mayfibrillate with serious or lethal consequences. This dilemma generallyhas been resolved in favor of continued pacing, although asynchronously,because the probability is greater that the patient will need pacingthan the patient will enter into ventricular fibrillation as a resultreceiving a pacing pulse during the vulnerable period. Accordingly, theappropriate noise mode generally includes asynchronous pacing with nosensing capability. When the pacemaker or ICD enters the noise mode, thesense circuit is disabled, or ignored, and the device paces at aconstant rate without regard to whether the heart is able to beat on itsown.

Although this may be an appropriate response to noise for some patients,asynchronous pacing without sensing may cause harm to other patients.This latter group of patients includes patients for which atachyarrhythmia may be induced by a pacing pulse that occurs during thevulnerable period of the cardiac cycle. For this group of patients, anasynchronous pacing mode of operation may result in the implantabledevice emitting a pacing pulse during the vulnerable period. If thiswere to happen, not only may the pacing pulse cause a tachyarrhythmia,but the arrhythmia may degenerate into a fatal fibrillation. However,because the sensing capability of the pacemaker or ICD is disabledduring the noise mode in conventional devices, the device is not able todetermine that the heart is experiencing the arrhythmia, and thus thedevice can respond with an appropriate antitachyarrhythmia pacingprotocol or, if necessary, a defibrillation pulse. Thus, the classicasynchronous pacing noise mode may induce a fatal tachyarrhythmia andfibrillation in the patient at a time when the patient cannot be rescuedby the delivery of a defibrillation shock. Although the probability ofcausing harm to a patient by pacing asynchronously during a high noiseevent is generally considered small, the possibility of harmnevertheless does exist and should be addressed.

Thus, there is a need for an implantable pacemaker or ICD to respond tothe presence of noise in a more appropriate manner than current devices.The new device should not put the patient at risk from suffering adangerous or fatal arrhythmia that is induced by the implanted deviceitself. The device preferably will able to determine the vulnerableperiod of each cardiac cycle during a high noise event and avoid pacingthe heart during that period of time.

SUMMARY OF THE INVENTION

Accordingly, there is herein provided an implantable medical device,such as a pacemaker or implantable cardioverter/defibrillator, thatelectrically stimulates the heart to beat. The medical device generallyincludes a processor, a plurality of electrodes, a sense amplifier, apulse generator, and a heart status monitor. The processor can determinewhen the patient has entered an environment of high electromagneticinterference. If, and when, the processor makes this determination, theprocessor forces the implantable device into a safe noise mode ofoperation. While in the safe noise mode (which preferably continues aslong as the patient is in the presence of the electromagneticinterference), the implantable device paces the heart on demand. Theprocessor determines when the heart needs to be paced by monitoring astatus signal from the heart status monitor. The status signal generatedby the heart status monitor is indicative of the physical orbio-mechanical state of the heart during a cardiac cycle and preferablyis not sensitive to the electromagnetic interference. Using the statussignal, the processor can determine the bio-mechanical state of theheart during a cardiac cycle and thus the vulnerable period, even in theface of high electromagnetic interference. Accordingly, the implantabledevice will pace the heart while experiencing a large EMI event butinhibit pacing during the vulnerable period.

The status signal generated by the heart status monitor is indicative ofsome aspect of the cardiac cycle. In accordance with the preferredembodiment, the heart status monitor includes a circuit used by theprocessor to determine the impedance between electrodes implanted in theheart or elsewhere in the patient's body. The status signal is thusindicative of the heart's impedance which varies in a periodic mannerwith each cardiac cycle. From the status signal, the processordetermines the heart's impedance as a function of time during a cardiaccycle. From that determination, the processor can ascertain when thevulnerable period occurs and inhibit pacing during the vulnerableperiod.

Rather than determining the heart's impedance, the heart status monitoralternatively may include a cardiac sensor that can be used to providethe status signal to the processor. The sensor may be any one of avariety of sensors that can track a cardiac parameter that variesaccording to a pattern from one cardiac cycle to another. Accordingly,the sensor may include a pressure transducer for providing an indicationof pressure inside one of the chambers of the heart. Further still, thesensor may include a volume transducer or flow transducer or any othersensor type that can generate a status signal from which the processorcan determine the beginning and ending of the vulnerable period.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings, wherein:

FIG. 1 is a schematic cut-away view of a human heart, in which thevarious relevant parts are labeled;

FIG. 2 is a portion of an exemplary surface electrocardiogram;

FIG. 3 is a schematic diagram of an implantablecardioverter/defibrillator constructed in accordance with the presentinvention and implanted in a human body;

FIGS. 4A-4G show waveforms representing various cardiac parameters thatvary with each cardiac cycle;

FIG. 5 is an exemplary block diagram of a preferred embodiment of theinvention as may be employed in the cardioverter/defibrillator shown inFIG. 3; and

FIG. 6 is an exemplary operational flow diagram of the preferredembodiment of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 3, an implantable medical device 100 constructedin accordance with the preferred embodiment is shown implanted andcoupled, in an exemplary configuration, to the patients heart by leads12, 14. The implantable medical device 100 may include a pacemaker,combination cardioverter/defibrillator and pacemaker, or any otherdevice that provides pacing support for the patient's heart. Forpurposes of describing the preferred embodiments of the invention,however, the implantable medical device 100 will hereafter be describedas an implantable cardioverter/defibrillator (“ICD”) with theunderstanding that pacemaker functions are also included. However, itshould be understood that the invention may be employed in any of avariety of implantable medical devices, including simple pacemakers.

The arrangement shown in FIG. 3 represents a dual chamber pacingconfiguration in which two leads 12 and 14 are coupled to a housing or“can” 101 of the ICD. In the configuration shown, the leads arepositioned in two chambers of the heart, lead 12 implanted in the rightventricle and the other lead 14 implanted in the right atrium. Each leadmay incorporate any desired number of electrodes. The leads 12,14 shownin FIG. 3, for example, are bipolar leads meaning each lead includes twoelectrodes. Lead 14 includes a tip cathode electrode 110 and a ringanode electrode 120. Lead 12 includes a tip cathode electrode 150 and ashock coil 140 for delivering a defibrillation shock pulse. Some ICDleads include tip and ring electrodes as well as a shock coil. As oneskilled in the art will understand, two, three, and four lead devicesthat have been used or suggested as various pacemaker or ICDconfiguration schemes in other applications may be employed in thepresent invention. Further, the ICD housing 101 itself can be used as anelectrode. The configuration shown in FIG. 3 is intended to be exemplaryonly of the many lead configurations possible for ICD 100.

ICD 100 may also communicate with an external programmer (notspecifically shown). If it is desired for the ICD 100 to include acommunication capability, any one of a number of communicationtechniques may be used. Preferably, however, the communication techniqueused involves wireless transmission of control and data signals such asthat disclosed by U.S. Pat. No. 5,314,453, incorporated here in byreference.

FIGS. 4A-4G illustrate that the bio-mechanical action of the ventriclesbegins to take place more or less at the same time that the electricalrepolarization of the ventricles is occurring. FIG. 4A includes thesurface ECG as in FIG. 2. Ventricular repolarization occursapproximately at the time of the T-wave on the surface ECG. FIGS. 4B-4Gillustrate several time-varying parameters each one indicative of someaspect of the heart during a cardiac cycle. The variation of theseparameters is illustrated generally over one cardiac cycle and thefigures are aligned in time with the surface ECG of FIG. 4A. FIG. 4Bshows the electrical impedance of the heart measured between a pair ofthe pacer's electrodes 110, 120, 140, 150. FIGS. 4C and 4D illustratethe venous pulse and heart sounds, respectively. The heart sounds areacoustic signals that are indicated as a type of resonant phenomena ofcardiac structures and blood as a consequence of one or more suddenevents in the heart (such as closure of a valve). FIG. 4E is the volumeof the ventricle. The blood flow through the aorta is shown in FIG. 4Fand the pressure of the left ventricle is shown in FIG. 4G. Thewaveforms of FIGS. 4B-4G are cyclical or periodic meaning they repeatwith each cycle of the patient's heart. Thus, each of the parameters inFIGS. 4B-4G can be correlated to the cardiac cycle. Accordingly, bymonitoring any one of these physical and electrical parameters, the ICD100 can determine the current state of the heart and moreover, determineor estimate the beginning and ending of the vulnerable period. Aspreviously mentioned, the vulnerable period is a period in which apacing pulse may itself cause life-threatening fibrillation.

The preferred embodiment of the ICD 100 is illustrated in the exemplaryblock diagram of FIG. 5. The ICD 100 generally includes a switch unit160, atrial and ventricular sense circuits 162, 164, a heart statusmonitor 165 which includes a processor 170 and an impedance circuit 166and/or a sensor 172, and a pulse generator 168. The exemplary embodimentof FIG. 5 shows ICD 100 with five electrodes, namely atrial tip and ringelectrodes 110 and 120, ventricular shock coil and tip electrodes 140,150, and can electrode 101. The invention, however, may be practicedusing any number of electrodes implanted in any chamber of the heart.

Referring still to FIG. 5, the atrial sense circuit 162 processessignals received from the atrial chamber of the heart via the atrialelectrodes 110, 120 and the ventricular sense circuit 164 processessignals from the ventricular chamber via the ventricular electrodes 140,150. The atrial and ventricular sense circuits 162, 164 generallyinclude a low power, highly sensitive amplifier, a band pass filter, anda threshold detector (not specifically shown). The amplifier amplifiesthe electrical signal from the associated electrodes, and the band passfilter attenuates signals whose frequencies are outside the range offrequencies known to correspond to cardiac signals. The thresholddetector compares the amplified and filtered signal to a referencesignal to determine when a cardiac event (also referred to as a “senseevent”) has occurred. If the magnitude of the amplified and filteredcardiac signal exceeds the reference signal, the processor 170determines that a sense event has occurred. The processor 170 may thenpace the heart based either on detecting or not detecting sense events.For example, the processor 170 may initiate a ventricular pacing pulseif an atrial sense event has not been detected within a predeterminedperiod of time following a previous atrial sense event.

The pulse generator 168 is employed to produce an appropriate electricalpulse to stimulate the desired chamber of the heart to beat. Theprocessor 170 initiates the pulse generator 168 to produce a pacingpulse, and the pulse generator responds by delivering the pacing pulseto the desired chamber of the heart. The pulse generator may include arate limiter to prevent the processor 170 from erroneously pacing theheart at an excessively high rate.

Switch unit 160 preferably includes multiple solid state switches (notspecifically shown) and preferably one switch connects to eachelectrode. The states of the switches are controlled by processor 170via control lines 169. The processor 170 controls the state of switchunit 160 to connect the electrodes to either the sense circuits 162, 164or the pulse generator 168. Further, the processor 170 may control thestate of each switch contained within switch unit 160 independently ofother switches that may be contained within switch unit 160. Theprocessor 170 connects the desired electrodes (either the atrialelectrodes 110, 120 or the ventricular electrodes 140, 150) to pulsegenerator 168 when the processor desires to initiate a pacing pulse tothe appropriate atrial or ventricular chamber. The processor 170 mayalso be employed to sense the electrical activity in either the atrialor ventricular chambers, or both, by altering the state of switch unit160 to connect the desired electrodes to the corresponding sense circuit162, 164.

The ICD 100 preferably operates in a predetermined pacing mode that issuitable for the patient. Accordingly, ICD 100 may be programmed tooperate in one of a number of pacing modes. For example, the ICD 100 maybe programmed to sense electrical activity in the atrium, and then topace the ventricle following a predetermined time delay after the atrialsense event if the ventricle has not beat on its own.

The ICD 100 can determine when it is in a high noise environment such asmay occur when the patient is near a source of EMI. As previouslymentioned, this high noise condition may occur as the patient walksthrough a metal detector or is near a radio transmitter, welder,security surveillance system, etc. The EMI from such a source mixes inwith and thus distorts, the naturally occurring cardiac signal beingmonitored by the processor 170 by one or both of the sense circuits 162,164. The processor 170 can determine when the pacer 100 is experiencingexcessive EMI using any one of a number and of processing techniquessuch as that described in U.S. Pat. Nos. 5,010,887, 4,516,579 and5,697,958, incorporated herein by reference.

Referring still to FIG. 5, if the processor 170 determines that the ICD100 is in a high noise environment, the processor 170 preferably changesthe mode of operation to a safe noise mode. In the safe noise mode ofthe preferred embodiment, all intracardiac electrogram sensing activityis terminated, and the electrogram sense circuits 162, 164 are turnedoff by switch unit 160 and the processor 170. Alternatively, processor170 may be programmed to ignore the output signals of sense circuits162, 164 during the period that high EMI is detected. As used herein,the term or phase “discontinues monitoring” as applied to the processorincludes either way to terminate electrogram sensing. Rather thanswitching to an asynchronous pacing mode during periods that high EMI ispresent, as do prior art implantable devices, ICD 100 instead switchesto an inhibited, safe noise pacing mode. In the preferred safe noisepacing mode, the processor 170 continues to pace the heart if the heartneeds to be paced (i.e., on demand). However, rather than monitoring theelectrical activity of the heart via the sense circuits 162, 164, theprocessor 170 determines the bio-mechanical state of the heart duringthe cardiac cycles using an alternative technique described below.

The alternative heart monitoring technique uses a heart status monitor165 to determine the bio-mechanical state of the heart. According to thepreferred embodiment, the heart status monitor includes impedancecircuit 166. The impedance circuit 166 generally processes theelectrical signal from a ventricular electrode and provides an outputstatus signal to the processor 170. The processor 170 uses the statussignal from the impedance circuit 166 to compute the impedance of theheart. As shown in FIG. 4B, the impedance of the heart varies in arhythmic manner with each cardiac cycle. The impedance waveform of FIG.4B reaches a peak shortly after the T-wave of the surface ECG. Theprocessor 170 can determine when the heart is in the vulnerable periodof the cardiac cycle by computing and tracking the heart's impedance.The impedance circuit 166 can be any suitable circuit, such as thatdescribed in U.S. Pat. No. 5,531,772, incorporated herein by reference.The impedance measurement technique described in U.S. Pat. No. 5,531,772is particularly beneficial because it is not very sensitive to EMI orother sources of noise.

Alternatively or additionally, the heart status monitor may include asensor 172 to provide an indication of the bio-mechanical activity ofthe heart. Sensor 172, for example, may be a pressure transducer, a flowtransducer, an accelerometer, a sound transducer, or any other type ofdevice or combination of devices that provides a status signal via line173 from which the processor 170 can determine the current state of theheart during each cardiac cycle. The sensor 172 should be one that isnot susceptible to EMI. For example, the sensor 172 may comprise apressure transducer that is incorporated into the distal end of one orboth of the leads 12, 14 (i.e., the end implanted in the heart itself).Such a pressure transducer can provide an electrical signal to theprocessor 170 from which the processor can compute the atrial orventricular pressure (as exemplified in FIG. 4G). If sensor 172comprises a flow transducer or a volume transducer, the processor 170can compute aortic blood flow as in FIG. 4F or ventricular volume as inFIG. 4E, respectively. In general, the ICD 100 may include any sensorfrom which the bio-mechanical state of the heart can be determinedduring a high noise event.

The operation of the ICD 100 is described generally with respect to theflow diagram 200 of FIG. 6. In step 202, the ICD 100 enters or remainsin its normal, predetermined pacing mode suitable for the patient.Preferably, the normal pacing mode includes demand pacing based on theelectrical activity of the heart at the electrodes. In step 204, if theICD 100 is not in a high noise environment, the ICD 100 operationallyremains in its normal pacing mode (step 202). However, if a high noiseenvironment is detected in step 204, the processor 170 changes modes tothe safe noise mode of the preferred embodiment (step 206). Thepreferred safe noise mode includes demand pacing that is inhibited inthe vulnerable period which is determined from the heart's impedance orother cardiac-related parameters as described above. If it is determinedthat the heart is in the vulnerable period, processor 170 (FIG. 5) willnot signal pulse generator 168 to provide a pacing pulse. In step 208,the processor 170 determines whether the ICD 100 is still in a highnoise environment. If so, the ICD remains in the safe noise mode ofoperation to prevent the ICD form itself causing fibrillation. If not,the ICD 100 reverts back to its normal pacing mode of step 202.

Thus, the implantable device described above provides a significantimprovement over the prior art. As explained, the invention permitsdemand-type pacing to continue during a large amplitude EMI event. Thus,the risk of pacing during the vulnerable period is much less than withprior art devices.

While preferred embodiments of this invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit or teaching of this invention. Theembodiments described herein are exemplary only and are not limiting.Many variations and modifications of the system and apparatus arepossible and are within the scope of the invention. Accordingly, thescope of protection is not limited to the embodiments described herein,but is only limited by the claims which follow, the scope of which shallinclude all equivalents of the subject matter of the claims.

What is claimed is:
 1. A method for electrically stimulating a patient'sheart to beat, comprising: pacing the heart in a first operational mode;determining when the patient is in an environment characterized by highelectromagnetic interference; switching to a second operational modewhen it is determined that the patient is in a high electromagneticinterference environment; and pacing the heart in said secondoperational mode on demand.
 2. The method of claim 1, wherein pacing theheart in said second operational mode includes determining the impedanceof the heart.
 3. The method of claim 2, wherein pacing the heart in saidsecond operational mode further includes determining when the heartneeds to be paced by monitoring the impedance of the heart.
 4. Themethod of claim 3, wherein pacing the heart in said second operationalmode further includes determining a vulnerable period for a cardiaccycle based on said impedance.
 5. The method of claim 4, wherein pacingthe heart in said second operational mode includes pacing the heart ondemand, but not during said vulnerable period.
 6. The method of claim 4,wherein pacing the heart in said second operational mode furtherincludes determining when said heart needs to be paced by monitoring aheart status signal.
 7. The method of claim 6, wherein pacing the heartin said second operational mode includes determining when said heartneeds to be paced by monitoring a heart status signal provided by acardiac sensor.
 8. The method of claim 7, wherein pacing the heart insaid second operational mode includes determining when said heart needsto be paced by monitoring a heart status signal provided by a pressuresensor.
 9. The method of claim 7, wherein pacing the heart in saidsecond operational mode includes determining when said heart needs to bepaced by monitoring a heart status signal provided by a volume sensor.10. The method of claim 7, wherein pacing the heart in said secondoperational mode includes determining when said heart needs to be pacedby monitoring a heart status signal provided by a flow sensor.
 11. Themethod of claim 6, further includes wirelessly transmitting the heartstatus signal to an external programmer.
 12. The method of claim 1,wherein stimulating a patient's heart to beat comprises stimulating ananimal's heart to beat.
 13. A method of providing pacing for a heart,comprising: pacing the heart in a first operational mode; detecting thepresence of noise; pacing the heart on demand when the noise isdetected, wherein pacing on demand includes: discontinuing pacing theheart in the first operational mode; pacing the heart based on signalsreceived from the heart; detecting a vulnerable period; discontinuingpacing during the vulnerable period; and resuming pacing on demand atthe end of the vulnerable period; and resuming pacing the heart in thefirst operational mode in the absence of noise.
 14. The method of claim13, wherein pacing on demand includes determining the impedance of theheart.
 15. The method of claim 14, further includes determining when theheart needs to be paced by monitoring the impedance of the heart. 16.The method of claim 15, wherein determining when said heart needs to bepaced includes determining when said heart needs to be paced bymonitoring a heart status signal.
 17. The method of claim 16, whereindetermining when said heart needs to be paced includes determining whensaid heart needs to be paced by monitoring a heart status signalprovided by a cardiac sensor.
 18. The method of claim 16, whereindetermining when said heart needs to be paced includes determining whensaid heart needs to be paced by monitoring a heart status signalprovided by a pressure sensor.
 19. The method of claim 16, whereindetermining when said heart needs to be paced includes determining whensaid heart needs to be paced by monitoring a heart status signalprovided by a volume sensor.
 20. The method of claim 16, whereindetermining when said heart needs to be paced includes determining whensaid heart needs to be paced by monitoring a heart status signalprovided by a flow sensor.
 21. The method of claim 13, further includeswirelessly transmitting the signals received from the heart to anexternal programmer.